From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <chasm@rift.sytes.net>
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39])
	by yquem.inria.fr (Postfix) with ESMTP id BD896BB91
	for <caml-list@yquem.inria.fr>; Fri, 17 Dec 2004 03:10:07 +0100 (CET)
Received: from galileo.rift (host217-44-198-112.range217-44.btcentralplus.com [217.44.198.112])
	by concorde.inria.fr (8.13.0/8.13.0) with ESMTP id iBH2A7IJ013976
	for <caml-list@yquem.inria.fr>; Fri, 17 Dec 2004 03:10:07 +0100
Received: from chasm by galileo.rift with local (Exim 3.35 #1 (Debian))
	id 1Cf7ZG-0001Vp-00
	for <caml-list@yquem.inria.fr>; Fri, 17 Dec 2004 02:10:06 +0000
Subject: Threaded ocaml code, native threads, and linux SMP
To: caml-list@yquem.inria.fr
Date: Fri, 17 Dec 2004 02:10:06 +0000 (GMT)
X-Mailer: ELM [version 2.4ME+ PL66 (25)]
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Message-Id: <E1Cf7ZG-0001Vp-00@galileo.rift>
From: Julian <chasm@rift.sytes.net>
X-Miltered: at concorde with ID 41C23FFF.000 by Joe's j-chkmail (http://j-chkmail.ensmp.fr)!
X-Spam: no; 0.00; ocaml:01 threads:01 wrote:01 ocaml:01 rec:01 ocamlopt:01 -thread:01 cmxa:01 threads:01 cmxa:01 wrote:01 incorrectly:01 unix:01 pair:01 pair:01 
X-Spam-Checker-Version: SpamAssassin 3.0.0 (2004-09-13) on yquem.inria.fr
X-Spam-Status: No, score=0.4 required=5.0 tests=DNS_FROM_RFC_ABUSE,
	FORGED_RCVD_HELO autolearn=disabled version=3.0.0
X-Spam-Level: 

I wrote the following CPU-bound function in order to test the
Thread module in ocaml:

    let rec loop pair = match pair with
              (100000, 50000) -> ()
            | (     i, 50000) -> loop (i+1, 0)
            | (     i,     j) -> loop (i, j+1) ;;

This function was used in two test programs:

* Version 1 *

    let t1, t2 = Thread.create loop (0,0),
                 Thread.create loop (0,0) ;;

    Thread.join t1;;
    Thread.join t2;;


* Version 2 *

    loop (0,0);
    loop (0,0);;


The test programs were each compiled with the following command:
(ocaml 3.08, debian sid, linux kernel 2.6):

    ocamlopt -thread -o <name> unix.cmxa threads.cmxa <name>.ml


I then tried running the two versions on an SMP machine. I found that
whereas I expected version 1 to run roughly twice as fast, they actually
took the same amount of time.

Closer inspection using "ps" revealed that only one processor was being
used. When I wrote a similar test program in C, calling the pthread
functions directly, the threads were run on separate CPUs as expected.

The "ldd" command reveals that the C version and the ocaml version were
relying on the same native pthread library.

So why doesn't the ocaml version use 2 processors?
Is it a flaw with my program?
Did I compile the program incorrectly?


Thanks for any help you can give,
Julian